1. Filed of the Invention
The present invention relates to an inverter control device for use with a driving device such as a motor.
2. Description of the Related Art
FIG. 6 shows an example of an inverter and a conventional system structure of the inverter control device. An inverter 11 for driving a motor 1 comprises transistors 2a, 2b, 2c, 2d, 2e, and 2f including an emitter and a collector, and any of which is connected to a direct-current power supply. Each base of the transistors is connected to output signals from comparators 4a, 4b, 4c, 4d, 4e, or 4f to open or close a flow of current. According to output voltage command values eu*, ev*, and ew* of each phase which is commanded, a duty ratio is controlled, as follows, in comparison with a reference wave signal by comparators 5a, 5b, and 5c. Namely, the output voltage command values eu*, ev*, and ew* which are input to the comparators 5a, 5b, and 5c are compared to a triangular wave signal output from a reference wave generating circuit 9 to make a pulse signal. As the duty ratio of the pulse signal is directly proportional to the output voltage command values eu*, ev*, and ew*, an average value of the voltage applied to the motor 1 is also directly proportional to the output voltage command values eu*, ev*, when the triangular wave signal is high.
In this conventional inverter control device, for example, when the transistors 2a and 2b that make up the inverter 11 are simultaneously turned on, a direct current power supply 3 is short-circuited, and there is a possibility of damage to the circuit due to the flowing of the excessive current. To preventing such a problem, in an example as shown in FIG. 6, an idle time is commonly given to each ON signal of the paired transistors 2a and 2b by circuit elements so as to control to have a predetermined interval in the control of the ON signal of the inverter, with the circuit elements comprising resistors 6a, 6b, and 6c, capacitors 7a, 7b, and 7c, and resistors 8a, 8b, and 8c. This interval is generally called a deadtime, and the deadtime has been produced by a deadtime producing circuit 10 consisting of the comparators 4a, 4b, 4c, 4d, 4e, and 4f, and the above circuit elements.
Existence of the deadtime allows the output voltage of the inverter to generally become nonlinear against the output voltage command values eu*, ev*, and ew* which are input. In an example of a u-phase, the relationship between the output voltage command value and an actual output voltage eu can be approximated by equation (1). EQU eu=eu*-ed(iu) (1)
Where iu represents an output current value of the inverter, and ed(iu) is a function of iu presented with equation (2). The function represented with equation (2) is shown in FIG. 8. ##EQU1## Where edu represents a constant determined by the deadtime, and iu# is a fixed constant determined by the time constant of the motor windings.
FIG. 7 illustrates an example of a system structure of a conventional inverter control device for the purpose of correcting nonlinearity between a command value and an output by means of such deadtime. This inverter control device uses the relationship of equation (1) and controls the inverter by applying a correction command value, which is added with the deadtime compensation represented by equation (2), to the voltage command value, and performs deadtime correction. In this device, a deadtime compensation computing unit 17 computes deadtime compensation amounts ed(iu), ed(iv), and ed(iw) on the basis of phase current detecting values iu, iv, and iw, and equation (2). On the other hand, subtracters 13a, 13b, and 13c compute a current error between an current command value and an actual output current value so that a current control computing unit 15a, 15b, and 15c may output the voltage command values eu*, ev*, and ew* according to the current error. Adders 16a, 16b, and 16c add the voltage command values eu*, ev*, and ew* to the respective deadtime compensation amounts euc*, evc*, and ewc*, and output euc*, evc*, and ewc* as correction voltage command values. In an example of u-phase, the relationship between the phase voltage command value and the correction voltage value may be represented with the following equation (3). EQU euc*=eu*+ed(iu) (3)
Where, as mentioned above, in a device as shown in FIG. 7, the fixed constant is iu# and edu in equation (2) can be used to find the deadtime compensation amount ed(iu) of equation (3). This correction voltage command value is input to a PWM (pulse-width-modulated) circuit 12, and transistors 2a, 2b, 2c, 2d, 2d, 2e, and 2f are turned on or off on the basis of the correction voltage command. It is known that the PWM circuit 12 is used of the same circuit as the inverter control circuit shown, for instance, in FIG. 6. Current detectors 14a, 14b, and 14c shown in FIG. 7 are examples of such detectors for detecting the output current iu to be applied in equations (2) and (3).
Since in a conventional inverter control device as shown in FIG. 7, there exists deadtime, the output voltage of the inverter is nonlinear against the output voltage command which is input and an accurate output voltage can not be obtained. FIG. 9(A) represents the voltage command and the reference wave of the u-phase shown in FIG. 6, in which the triangular wave, transistors 2a and 2b, and the waveform of terminal voltage of the u-phase are shown respectively. Here, Td shown in FIG. 9 indicates deadtime duration, V a plus voltage of the direct-current power supply 3 in FIG. 6, and -V a minus voltage. As shown in FIG. 9, in such a conventional device, when the polarity of the output current of the inverter of FIG. 9(E) is positive, the terminal voltage of the u-phase is V while the transistor 2a in FIG. 9(C) is ON, and, in contrast, when a polarity of the output current in FIG. 9(E) is minus, the terminal voltage of the u-phase is V when the transistor 2b in FIG. 9(D) is OFF. Moreover, when the output current of the inverter in FIG. 9(G) is zero, the terminal voltage of the u-phase is either V when the transistor 2a is ON or -V when the transistor 2b is ON, and the terminal voltage of the u-phase is at a floating level during the deadtime in which both of the transistors 2a and 2b are OFF. Accordingly, an accurate output voltage can not obtained because the output voltage of the inverter is nonlinear versus the input voltage command value, at a changing point of the current polarity, in other words, at neighborhood of the zero point of the output current.
In a conventional inverter control device as shown in FIG. 7, a nonlinear element of the output voltage against the voltage command according to the deadtime is approximated and corrected by applying equation (2). Here, iu# and edu in equation (2) are constant, but, in reality, the value of iu# is changed as the time constant of the motor windings changes due to heating of the motor or the like. The value of edu is not always limited to only one due to a variety of switching devices. The edu error occurs because the value of edu is changed by the heating of the switching devices. Strictly speaking, it may not be approximated by equation (2) when the output current iu is in the range of -iu#&lt;iu&lt;iu#. Since the error of the deadtime compensation amount occurs by these factors, accurate deadtime compensation can not be performed. Accordingly, the conventional device has such a problem that it is hard to accurately control the output current of the inverter.